JAKARTA — A previously unknown population of rare orangutans has been discovered in the forests of Indonesian Borneo, raising hopes for the species' survival, conservationists said Monday.

Up to 5,000 endangered Bornean orangutans are believed to be living in limestone mountains in East Kalimantan province after surveyors in December found 219 orangutan "nests", Nature Conservancy scientist Erik Meijaard told AFP.

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The nests, sleeping platforms made of branches and leaves suspended in the trees, indicate there could be "several hundred to several thousand" orangutans living in the 2,500 square kilometre (965 square mile) area, Meijaard said.